The present invention relates to yellow iron oxide hydroxide particles and a process for producing the particles, and more particularly, to yellow iron oxide hydroxide particles not only having an excellent dispersibility and a further improved heat resistance but also exhibiting less change in hue between before and after treatment for improving the heat resistance, and a process for producing such particles.
Yellow pigments have been extensively used in the form of dispersion in resins or vehicles as color pigments for the production of resin compositions, paints, printing inks and asphalt for road construction. Since yellow color is effective for indicating restraints, warnings or cautions in traffic control, the yellow pigments have been especially widely used to color pigments for asphalt for road construction or paints for traffic control signs (traffic paints).
These yellow pigments are required to exhibit not only an excellent dispersion in resins or vehicles but also an excellent heat resistance.
As conventional yellow pigments, there have been extensively used lead chromate, strontium chromate, cadmium sulfide, iron oxide hydroxide or the like.
The above-mentioned lead chromate, strontium chromate and cadmium sulfide have been widely used as color pigments for resins, paints, printing inks, etc., because of an excellent heat resistance thereof. However, it is known that these conventional yellow pigments exhibit toxicity and carcinogenicity. Therefore, there has been a strong demand for alternate yellow pigments from the viewpoints of hygiene and safety as well as prevention of environmental pollution.
On the other hand, the iron oxide hydroxide particles are nontoxic yellow pigments and, therefore, more suitable from the standpoints of not only hygiene and safety but also prevention of environmental pollution. However, there is a problem that the iron oxide hydroxide particles is deteriorated in heat resistance.
Specifically, since the iron oxide hydroxide particles has a chemical composition of Fe.sub.2 O.sub.3.nH.sub.2 O, i.e., contain crystallization water therein, dehydration from the particles is generally initiated at about 200.degree. C. as the heating temperature is elevated. When the heating temperature is further elevated, the iron oxide hydroxide particles are transformed into reddish brown hematite (.alpha.-Fe.sub.2 O.sub.3) at about 230.degree. C.
For this reason, it is difficult to use the iron oxide hydroxide particles as a color pigment of thermoplastic resins such as polyethylene, polypropylene, styrene polymers, polyamides, polyolefins or ABS resins which are usually molded at as high a temperature as not less than 200.degree. C., or traffic paints which are subjected to heating and melting treatments at a temperature of 200 to 260.degree. C. upon use thereof.
In order to improve the heat resistance of the iron oxide hydroxide particles, various treatments have been proposed or attempted. However, since such iron oxide hydroxide particles show a large change in hue between before and after these treatments, it becomes difficult to attain a good product design from the viewpoint of hue which is essential and critical to color pigments. Accordingly, there has been a demand that the change in hue of the iron oxide hydroxide particles between before and after these heat resistance-improving treatments is as small as possible.
As various methods proposed of attempted to improve the heat resistance of iron oxide hydroxide particles, for example, hitherto, there have been proposed (1) a method of subjecting iron oxide hydroxide particles to hydrothermal treatment (autoclaving treatment) in water or in an aqueous alkaline solution using an autoclave (Japanese Patent Publication (KOKOKU) No. 53-28158(1978), etc.); (2) a method of coating surfaces of iron oxide hydroxide particles with an aluminum compound, a silicon compound or the like, a method of incorporating these compounds into the iron oxide hydroxide particles in the form of a solid solution thereof (Japanese Patent Publication (KOKOKU) No. 6-17237(1994), Japanese Patent Application Laid-open (KOKAI) No. 9-16553(1997), etc.); (3) a method using the combination of the above-mentioned methods (1) and (2) (Japanese Patent Publications (KOKOKU) Nos. 49-16531(1974), 54-7293(1979) and 55-8462(1980), Japanese Patent Application Laid-open (KOKAI) No. 57-57755(1982), etc.); or the like.
More specifically, in Japanese Patent Application Laid-open (KOKAI) No. 9-165531 (which was laid-open on Jun. 24, 1997), there is described a process for producing heat-resistant yellow iron oxide hydroxide particles, which comprises heat-treating iron oxide hydroxide particles in an aqueous alkaline solution having a pH value of not less than 10, followed by filtering and washing with water, thereby reducing the content of a soluble sulfate in the iron oxide hydroxide particles to not more than 2,000 ppm (calculated as SO.sub.4); heat-treating the yellow iron oxide hydroxide particles in an aqueous acid solution having a pH value of not more than 4, followed by filtering and washing with water, thereby reducing the content of a soluble sodium salt in the yellow iron oxide hydroxide particles to not more than 1,000 ppm (calculated as Na), thereby obtaining a high purity iron oxide hydroxide particles containing less amounts of the soluble sulfate and the soluble sodium salt; adjusting the pH value of a water dispersion containing the high purity iron oxide hydroxide particles to not less than 10 or not more than 4; adding an aluminum compound to the water dispersion, followed by stirring; and adjusting again the pH value of the resultant water dispersion to 5 to 9 to deposit a hydroxide of aluminum on surfaces of the high purity iron oxide hydroxide particles.
In Japanese Patent Publication (KOKOKU) No. 54-7293(1979), there is described a process for producing a stabilized iron oxide pigment, which comprises hydrothermal-treating (autoclaving) an alkaline aqueous slurry containing iron oxide pigment particles at not more than 250.degree. C., and treating the slurry under ordinary pressure in the presence of fine oxide hydroxide of at least one element selected from the group consisting of zirconium, titanium, aluminum and antimony to deposit the oxide hydroxide on surfaces of the pigment particles.
In Japanese Patent Publication (KOKOKU) No. 55-8462(1980), there are described a heat-resistant yellow iron oxide hydroxide pigment containing AlOOH in the form thereof and, and a process for producing a heat-resistant yellow iron oxide hydroxide pigment containing AlOOH in the form thereof and, which comprises adding alkali to an aqueous solution of ferrous salt to form a colloidal precipitate; and after aging the colloidal precipitate, adding thereto a water-soluble or alkali-soluble aluminum compound and a water-soluble silicate, tin salt or zinc salt to conduct a hydrothermal treatment.
In Japanese Patent Application Laid-open (KOKAI) No. 57-57755(1982), there is described a method of improving properties of yellow iron oxide hydroxide pigment, which comprises dispersing yellow iron oxide hydroxide (having a chemical structure of .alpha.-FeOOH) in water or a 10N or less-alkali aqueous solution to conduct a hydrothermal treatment at 100 to 250.degree. C.; and adding at least one of an oxide, an oxide salt and a hydroxide of silicon, aluminum or magnesium to the dispersion to conduct a hydrothermal treatment again at 100 to 250.degree. C.
Presently, most demanded yellow iron oxide hydroxide particles are those not only having an excellent dispersibility and an enhanced heat resistance but also exhibiting less change in hue between before and after treatment for improving the heat resistance thereof. However, such yellow iron oxide hydroxide particles satisfying all of the above-mentioned properties have been still unavailable until now.
That is, in the case of the yellow iron oxide hydroxide particles treated by the above-mentioned method (1), although the heat resistance thereof is improved, the configuration and particle size distribution of the particles are considerably changed between before and after the heat resistance-improving treatment, resulting in large change in hue of the particles therebetween.
In the case of the yellow iron oxide hydroxide particles treated by the above-mentioned method (2), although the change in hue between before and after the heat resistance-improving treatment is restricted to a small level, the dispersibility and the heat resistance of the obtained particles are unsatisfactory because yellow iron oxide hydroxide particles as agglomerated are coated with an aluminum compound or the like.
Further, in the case of the yellow iron oxide hydroxide particles treated by the above-mentioned method (3), although the heat resistance is improved, the change in hue between before and after the heat resistance-improving treatment becomes considerably large similarly to those treated by the method (1).
The present inventors have already successfully produced yellow iron oxide hydroxide particles having not only an excellent dispersibility and an enhanced heat resistance but also less change in hue between the heat resistance improving treatment (Japanese Patent Application No. 7-348047(1995) corresponding to Japanese Patent Application Laid-open (KOKAI) No. 9-165531(1997)).
However, it has been strongly demanded to still further improve properties of such yellow iron oxide hydroxide particles, especially dispersibility and heat resistance thereof, which exert significant influences on not only workability but also quality and functions of the yellow iron oxide hydroxide particles.
As a result of the present inventors' earnest studies, it has been found that by adding an aluminum compound and a ferrous salt compound to a water dispersion containing iron oxide hydroxide particles such that the amounts of the aluminum compound and ferrous salt compound added are 0.1 to 10% by weight (calculated as Al) and 0.1 to 50% by weight (calculated as Fe), respectively, based on the weight of the iron oxide hydroxide particles, followed by mixing together, and passing an oxygen-containing gas through the water dispersion to deposit a composite oxide hydroxide comprising Fe and Al on surfaces of the iron oxide hydroxide particles, the obtained yellow iron oxide hydroxide particles not only have an excellent dispersibility and an enhanced heat resistance but also exhibit less change in hue from that before the heat resistance-improving treatment (i.e., less change of a hue from comparison of the hue of yellow iron oxide hydroxide particles between before and after deposition of the composite oxide hydroxide thereon). The present invention has been attained on the basis of the finding.